Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.
Optical networks often employ wavelength division multiplexing (WDM) to increase transmission capacity. In a WDM network, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
The maximum distance that a signal can be transmitted in a WDM or other optical network without amplification is limited by absorption, scattering and other loss associated with the optical fiber. To transmit signals over long distances, optical networks typically include a number of discrete amplifiers spaced along each fiber route. The discrete amplifiers boost received signals to compensate for transmission losses in the fiber.
A problem with optical amplifiers is that signals accumulate a number of nonlinear impairments along the length of the fiber. The source of these impairments for WDM and other systems in which a plurality of optical channels are transmitted on the same optical fiber include cross-talk between channels that occurs during transmission or incomplete channel selection by the receiving terminal. To account for these impairments, WDM systems typically employ 50 gigahertz (GHz) spacing between 10 gigabits per second (Gb/s) channels. This channel spacing allows a number of channels to be transmitted per fiber and thus increases the capacity of the network at the cost of decreasing the ability of optical receivers to discriminate between the channels. As a result, cross talk between channels is increased and transmission distances between regeneration limited.